American Kevin Stadler celebrated Independence Day by claiming a three shot halfway lead in the Alstom Open de France, despite a shaky finish to his second round.

Stadler carded a flawless opening 64 at Le Golf National and did not drop his first shot of the tournament until his 32nd hole at the 2018 Ryder Cup venue.

But after bouncing back in style with birdies on the next two holes, the World Number 62 shanked his approach to the par five ninth - his final hole - to run up a double bogey seven.

"I just tried to get a little too much out of it," admitted Stadler, who had to settle for a round of 68. "I didn't have the best of lies and had to hit the shot perfectly. I got a little too steep on it and it came off the hosel and into the deep stuff.

"It was not exactly how I planned to finish the day but overall it was another good day so it's all good. I putted brilliantly the first couple of days. I got a little sloppy on my final nine today but other than that struck the ball very well as well, had a lot of chances and rolled in a lot of nice putts."

Stadler won his maiden US PGA Tour title in February and was half of the first father-son pairing to play in the same Masters Tournament in April with his father Craig, the 1982 champion.

The 34 year old finished joint eighth at Augusta National but that has been his only top-ten since winning in Phoenix and he added: "It's been a little rotten the last couple of months but it's starting to show some signs of form again and I've hit plenty of good shots the last couple of days.

"Hopefully I can do more of the same at the weekend, the putter keeps co-operating and I am looking forward to the last couple of days."

Stadler's nearest challenger is Parisian Victor Riu, whose only bogey in a round of 67 came on the 18th after finding a greenside bunker, with 2012 winner Marcel Siem two shots further back on five under after a 72.

Riu has missed the cut in five of his last six events but some hard work with his coach and a touch of local knowledge paid off for the 29 year old.

"I'm attached to (the nearby) St-Nom-La-Bretéche but most of time I come here because the course is so good," Riu said. "I used to play here most of the time I'm in Paris. I didn't feel very good in my game but my coach arrived here on Monday. We are training very hard and I feel much, much better."

Defending champion Graeme McDowell added a 69 to his opening 70 to finish three under par - good enough for a share of fourth place with six other players - and was quick to acknowledge Stadler's superb performance.

"I said to (US Open Champion) Martin Kaymer it was nearly as good as his at Pinehurst, but not quite," said McDowell, whose winning total 12 months ago was nine under. "It's a great display of golf.

"It's Kevin's type of golf course and if he keeps playing like that he's going to be tough to catch, but I feel like three under is not a bad return for the last two days and if I can go low tomorrow and get myself back into it, who knows?"

Playing partner Kaymer, who won his second major title by eight shots last month, was a shot behind McDowell on two under after carding a 68 containing six birdies and three bogeys.